Lib Dem leadership frontrunners Sir Menzies Campbell and Simon Hughes have tackled Tony Blair over the quality of public services.
Acting leader Sir Menzies was barracked after choosing to highlight the number of head teacher posts filled by temps.
Mr Blair joked it was hard to fill jobs in "failing organisations".
Mr Hughes, widely tipped to stand, also got a boisterous reaction from MPs at prime minister's questions. MP Chris Huhne later said he might stand.
The economic spokesman and Eastleigh MP said he was "considering" the move but it was "premature" to say he would be a contender.
Nominations close on 25 January. Home affairs spokesman Mark Oaten and Sir Menzies are currently the only challengers.
Mr Hughes is due to reveal on Thursday whether he will stand.
Mr Oaten's camp released a letter on Wednesday from acting returning officer David Allworthy confirming he had the necessary support among MPs to stand for the leadership.
The letter, to Mr Oaten's agent Edward Lord, said: "I confirm that this
nomination is valid in accordance with the election regulations and rulings made under those regulations at the time of receipt."
'Byword for failure'
Earlier, Sir Menzies was allowed ask Mr Blair two questions in the Commons because he is acting party leader.
He said police were up in arms about reorganisation plans, a watchdog had said schools were failing a million pupils and NHS reforms had been branded as ill-judged by an MPs' committee.
He asked: "Why's the government making such a mess of public service reform?"
Blairism had become a become "byword for centralisation and a failure to deliver", he claimed.
But the prime minister pointed to improvements in schools and hospitals, arguing: "I think public service reform and investment is doing very well."
Mr Blair also shrugged off an attack from Mr Hughes, who asked why one NHS patient had needed to pay £10,000 for private treatment.
The prime minister said he accepted there were still people waiting too long, but said waiting lists had been cut and would be made shorter still.
And he seized on an aside from Mr Hughes, to say: "If he's going to back our reforms I'm going to start backing him rather than the other one."
25 January - nominations close
6 February - ballot papers sent out
1 March - vote closes
2 March - result announced
Looking round for Mark Oaten - currently Sir Menzies' only challenger - he added: "There is no end to the Lib Dem careers I could sacrifice this afternoon. Where's the other one?"
Mr Blair said the Lib Dems had to do some "hard thinking".
Mr Oaten toured the television studios rather than appear in the Commons chamber.
Announcing his candidature on Tuesday, home affairs spokesman Mr Oaten said he wanted to make the Lib Dems a "truly modern 21st century party".
"I'm really ambitious for this party," said the Winchester MP. "We have got to persuade and get across to this country that this party wants to get into power."
Mr Oaten is seen as on the right of the party and contributed to the Orange Book, which included essays by other MPs proposing a move to more free market policies.
In the spotlight
The 73,000 Lib Dem members only have until 1 March to make their choice - with ballot papers due to be sent out on 6 February.
The candidates will have their first chance to address Lib Dem members directly on Saturday at the party's "Meeting the challenge" conference in London.
Sir Menzies, 64, is believed to have the backing of more than a third of the party's 62 MPs and he has been endorsed by former leaders Lord Steel and Lord Ashdown.
The party's biggest donor Michael Brown, who gave the Lib Dems £2.4 million before the last election, said he almost certainly would not give the party more cash after its "despicable" treatment of Charles Kennedy, who resigned on Saturday, two days after admitting to a drink problem.
"I am very disappointed and disheartened about what has happened and how Charles Kennedy has been treated," he told BBC News.
"I really don't think I want to be involved in a party that treats their leaders and their members like this."
Mr Kennedy meanwhile has told his local paper he is not an alcoholic despite seeking help for his drinking.
He added: "I'm a social drinker, not a lone drinker."